Electrode for gaseous discharge devices



Jan. 19, 1932. C, H, THOMAS 1,842,215

ELECTRODE FOR GASEOUS DISCHARGE DEVICES Filed June 28, 1930 ATTO Patented Jan. 19, 1932 UNITED STATES PATENT OFFICE CHARLES HASTINGS THOMAS, `Ol' ELOOMFIELD, NEW JERSEY, ASSIGNOB TO WESTING- HOUSE LAMP COMPANY, A CORPORATION l' PENNSYLVANIA ELECTBODE FOB GASEOUS DISCHARGE DEVICES Application lcd June 28, 1930. Serial No. 464,485.

This invention relates to electric discharge devices and more particularly to electric discharge devices of the gaseous conduction type and to electrodes employed therein.

One of the objects of this invention is to increase the operating efficiency of gaseous conduction devices.

Another object of this invention is to :provide an electrode for such devices having a relatively low electrode drop in potential.

Another object of this invention is to provide a gaseous conduction device operable at relatively high electrode current densities with a relatively low electrode drop in poten- 1s tial.

Other objects and advantages will be apparent as the invention is more fully disclosed.

This invention is animprovement in the 2o invention disclosed in my copending application Serial No. 417,091 lfiled December 28, 1929, which application is assigned to the same assignee as the present invention. In

y said copending application I have disclosed the use in an electric discharge device of the gaseous conduction type of open ended hollow electrodes comprised at least in part of refractory thermionically active metal, speciically thorium. In such a type electrodethe cathode glow of the discharge is substantially confined to the hollow interior of the electrode and by reason of certain electron emitting properties of the thorium component of the electrode, the electrode drop in potential is materially lowered, thereby permitting the application of materially higher electrode current densities to the electrode without accompanying undesirable electrode sputtering effects.

With such open ended hollow electrodes the permissible size and depth of the opening therein is dependent upon the specific gases employed in the device, the pressure of the Y gas, and the desired operating potential across the electrodes, etc. l

In accordance with theobjects of the present invention the operating efliciency of this type electrode may be still further increased by coating the interior surface of the open 5 ended hollow electrode with thermionically active alkaline or alkaline earth metal compounds. y

Before further disclosing the nature of my invention reference should be had to the accompanying drawings wherein,

1g. is a perspective view of a gaseous conduction device of the positive column type embodymg the specific type electrodes of the present invention;

Fig. 2 is an enlarged cross-sectional view of the electrode, and

Fig. V3 is a voltage-current graph illustrating thebeneficial eiects obtained by the practice of the present invention as compared to those obtained heretofore with prior art electrodes.

Referring to Fig. 1 the discharge device is comprised of a long tubular glass envelope 1 which is relatively small in diameter with respect to its length, having enlarged ends 2 within which are enclosed electrodes 3 integral with support member 4 passing through press 4 to make electrical connection with current carrying conductors 6.

In the present illustration the narrow tubular portion 1 is shaped in the form of a letter N and the enlarged portions 2 are bent at right angles to the plane of the narrow tubular portion 1. Electrodes 3 are comprised of metal and in accordance with the present invention are hollow tubular inform, one end being closed, the open end 7 substantially facing the channel of the tubular portion of the device.

The electrode structure is shown in greater detail in Fig. 2 which is a cross sectional view of the same showing the hollow tubular feature in greater detail. Preferably the electrode is comprised of a solid cylindrical mass of metal and the recess portion 7 is obtained therein by drilling a hole therein of the roper or desired diameter to the proper or esired depth, depending upon the contemplated application or use of the electrode in gaseous conduction devices.

The particular size and depth of the hole and the overall dimensions `of the electrode and the speciic composition of the electrode will depend upon the specific device within which it is to be employed, the kindof gas 1 tion of the alkaline earth metal.

filling, the gas pressure and the current densities to be applied thereto. The inner surface of the electrode is coated with thermionically active alkaline earth metal compounds.

In the device illustrated in Fig. l the tubular shaped envelope is approximately .150 inches in diameter, and the gas filling is comprised preferably of neon at a pressure approximating 13.3 mm. of mercury. A common sized electrode which is useful over a Wide range of current densities in such a device is approximately .15 inches in diameter, about 5;/8 inches in length, and withln which a hole of about .O76 inches diameter to a depth of about l@ inch is drilled.

The electrode 3 is comprised preferably substantially of thorium, which may be prepared in accordance with the invention of copending application Serial No. 717,940, filed June 5. 1924, by J. W. Marden, et al., which application is assigned to the same assignee as the present invention. It may be comprised however of a refractory alloy of thorium or of a refractory metal base surfaced with or comprised at least in part of thorium.

In accordance with the practice of the present invention the interior hollow surface of the electrode is surfaced with a thermionically active alkaline or alkaline earth metal compound, specifically barium oxide, in any convenient manner, such as by repeatedly dipping the electrode in a barium nitrate solution and subsequently baking at elevated temperatures, or by applying the oxide as an aqueous or non-aqueous suspension to the surface. A thermally decomposable compound of barium may be employed if `desired. The exterior surface of the electrode may or may not be oxidized as is set forth in my copending application in order to more efliciently concentrate the glow discharge in the interior of the electrode.

The electrode thus coated is then mounted in any convenient manner upon the electrode support wire 4 and sealed into the glass envelope 1 of the device in the usual manner. The device is then exhausted, the electrodes 3 and envelope 1 being thoroughly freed of deleterious adsorbed and absorbed gases. During this exhaust operation the electrodes are incandcsced to elevated temperatures by high frequency induction means to effect thermal decomposition of the alkaline earth metal compounds to oxide compounds and further heated to effect at least in part activation of the electrode.

This activation of the electrode is believed to comprise substantially effecting at Lleast in part a thermal dissociation of the alkaline earth metal compounds and an interaction of the alkaline earth metal oxides or an alkaline earth met-al with the metal of thc electfrode element to effect the formation of a propor- This alkaline earth metal remains upon the electrode either as a component of the coating or as a surface film adherent to the base metal or alloyed at least in part therewith. Although the exact reaction involved is not clear the effect produced is believed to be due to the presence upon the electrode of metallic alkaline earth metal.

Following activation of the electrode and the complete removal of atmospheric gases, the desired monatomic or inert gas filling is introduced within the device and the device is sealed off' from the exhaust means in the customary manner.

The device is then given a preliminary seasoning operation during which an electrical discharge is maintained therein until the gas filling has been completely freed of deleterious residual atmospherical gases. During this seasoning stage the thorium component of the electrodes serve as the gas cleanup agent.

A gaseous conduction device constructed in accordance with the present invention incorporating drilled or open ended hollow thorium electrodes having the surface coated with barium oxide, activated and seasoned in accordance with the above disclosed process, may be operated at relatively high electrode current densities at relatively low electrode drop in potentials.

Referring to Fig. 3 of the accompanying drawings a graphic comparison of the operating characteristics of three types of electrodes operating in substantially the same pressure of an inert gas is clearly set forth. Curve I shows the type of operating curve obtained with solid thorium electrodes; curve II shows the type obtained with the drilled (or open ended hollow) electrodes of copending application Serial No. 417,091 above identified, which electrode for simplicity of identification has been labeled Drilled thorium cathode, and curve III illustrates the marked effect on the same type of drilled thorium electrode when coated with barium oxide and activated in accordance with the present invention.

It will be noted, for example, that when a current of 20 ma. is applied to the electrodes of such type that the electrode drop in potential of the electrode varies markedly. The solid thorium electrode has a potential'drop of approximately 150 volts; the drilled thorium electrode has a potential drop of about 140 volts; whereas the activated drilled thorium electrode has a potential drop of about 125 volts.

At 30 and 40 milliamperes current density the activated drilled thorium electrode has a potential drop not far different from the potential drop at 20 ma., and far below that of either the solid thorium or the drilled thorium electrode. This is believed due to the presence of the barium or other alkaline earth metals either within the coating composition, or superficially coating the electrode or alloyed therewith, depending upon the particular activated electrode theory that is applied.

While I have indicated in curve III that at higher electrode currents the voltage drop of the electrode rapidly lowers, it is to be appreciated that this does not give a measure of the satisfactory operating life of the electrode under such conditions. It is of course desirable toobtain a reasonably long operating life from the activated drilled thorium electrode vand this may only be obtained at the lower electrode current densities, below substantially two amperes per square decimeter electrode surface area. At electrode current densities materially greater than this the rate of evaporation or sputtering of the alkaline earth metal from the electrode due to positive ion bombardment is excessive and in due course the beneficial effect of the same is lost.

In calculating this current density I have taken overall surface of the electrode. With the drilled electrode of the size herein designated this overall surface approximates 260 square millimeters. It is to be specifically noted however that in employing a drilled electrode the glow concentrates within the recessed portion of the electrodeand it is estimated that approximately 90 to 95 per cent of the current is drawn from the recessed portion. When the exterior surface of the electrode is coated with insulating material the glow is concentrated to the interior and it is estimated that the maximum current density that may be safely applied to such an electrode without deleterious sputtering approximates 7'.0 ampers per squarev -decimeter of surface. v

Having broadly and specifically disclosed the nature and scope of the present invention it is apparent that many modifications and departures can be made from the specific embodiment hereinf set forth, but such modii cations and departures are anticipated as may fall within the scope of the following claims. c

What is claimed is:

l. In a gaseous discharge device an electrode comprised of an open ended hollow metal base element superficially coated with alkaline earth metal compounds, said base element being comprised atleast in part of thorium.

2. In a gaseous conduction device an electrode comprised substantially of an open ended hollow thorium base ielement superically coated with activated alkaline earth metal oxide compounds.

3. In a gaseous conduction device, an electrode comprised substantially of thorium supercially coated with thermionically active alkaline earth metal compounds.

4. A' aseousconduction discharge device comprising an 'enclosing glass envelope, a gaseous lling and at least one internally v disposed electrode, said electrode being comprised substantially of thorium, and one face of said electrode being recessed a substantial depth, the face of said recessed portion being surfaced at least in part with thermionically active alkaline earth metal compounds.

5. In combination an enclosing envelope, an inert gas filling, and at least one internally disposed electrode, said electrode being comprised substantially of an open ended hollow rare refractory metal body the recessed portion thereof bein superficially coated at least in part with activated alkaline earth metal compounds.

In testimony whereof I have hereunto subscribed my name this 25th da of June, 1930.

CHARLES HASTING THOMAS. 

